Transcript The Electromagnetic Field

The Electromagnetic Field
Maxwell Equations
Constitutive Equations
Boundary Conditions
• Gauss divergence theorem
Leads to
Boundary Conditions
Boundary Conditions
• The normal component of the magnetic
induction B is always continuous, and the
difference between the normal components
of the electric displacement D is equal in
magnitude to the surface charge density σ.
Boundary Conditions
• Stokes theorem
Leads to
Boundary Conditions
The tangential component of the electric field
vector E is always continuous at the b oundary
surface, and the difference between the
tangential components of the magnetic field
vector H is equal to the surface current density
K
Energy Density and Energy Flux
• The work done by the electromagnetic field
can be written as
• The right side can becomes
Energy Density and Energy Flux
• The equation can be written as
• Where U and S are defined as
Complex Numbers and
Monochromatic Fields
• For monochromatic light, the field vectors are
sinusoidal functions of time, and it can be
represented as a complex exponential
functions
Complex Numbers and
Monochromatic Fields
• a(t) can be also written as
Note: Field vector have no imaginary parts, only
real parts. The imaginary parts is just for
mathematical simplification.
Complex Numbers and
Monochromatic Fields
• By using the complex formalism for the field
vectors, the time-averaged Poynting’s vector
and the energy density for sinusoidally varying
fields are given by
Wave Equations and Monochromatic
Plane Waves
• The wave equation for the field vector E and
the magnetic field vector H are as follows:
Wave Equations and Monochromatic
Plane Waves
• Inside a homogeneous and isotropic medium,
the gradient of the logarithm of ε and μ
vanishes, and the wave equations reduce to
• These are the standard electromagnetic wave
equations.
Wave Equations and Monochromatic
Plane Waves
• The standard electromagnetic wave equations
are satisfied by monochromatic plane wave
• The wave vector k are related by
Wave Equations and Monochromatic
Plane Waves
• In each plane, k∙r =constant, the field is a
sinusoidal function of time. At each given
moment, the field is a sinusoidal function of
space. It is clear that the field has the same
value for coordinates r and times t, which
satisfy
ωt-k∙r = const
The surfaces of constant phases are often
referred as wavefronts.
Wave Equations and Monochromatic
Plane Waves
• The wave represented by
are called a plane wave because all the
wavefronts are planar.
For plane waves, the velocity is represented
by
Wave Equations and Monochromatic
Plane Waves
• The wavelength is
• The electromagnetic fields of the plane wave in
the form
• Where
and
are two constant unit vector
Wave Equations and Monochromatic
Plane Waves
• The Poynting’s vector can be written as
• The time-averaged energy density is
Polarization States of Light
• An electromagnetic wave is specified by its
frequency and direction of propagation as well
as by the direction of oscillation of the field
vector.
• The direction of oscillation of the field is
usually specified by the electric field vector E.